Add support for a backdoor driver option that enables emitting header

usage information in JSON to a file

Each line in the file is a JSON object that has the name of the main
source file followed by the list of system header files included
directly or indirectly from that file.

For example:

{"source":"/tmp/foo.c",
 "includes":["/usr/include/stdio.h", "/usr/include/stdlib.h"]}

To reduce the amount of data written to the file, only the system
headers that are directly included from a non-system header file are
recorded.

In order to emit the header information in JSON, it is necessary to set
the following environment variables:

CC_PRINT_HEADERS_FORMAT=json CC_PRINT_HEADERS_FILTERING=only-direct-system

The following combination is equivalent to setting CC_PRINT_HEADERS=1:

CC_PRINT_HEADERS_FORMAT=textual CC_PRINT_HEADERS_FILTERING=none

Differential Revision: https://reviews.llvm.org/D137996